This invention relates to a technology of reducing the frictional resistance of a ship by ejecting bubbles on a submerged surface of a ship.
A technology relating to the friction reducing ship noted above is disclosed in Japanese Patent Applications, First Publication, Sho 50-83992, Sho 53-136289, Sho 60-139586, Sho 61-71290, and in Practical Utility Models, Sho 61-39691 and Sho 61-128185. The friction reducing ship achieves reduction in frictional resistance by introducing numerous micro-bubbles at the surface of the outer hull plate of a ship by jetting into the water a gaseous substance such as air from the ship outer hull plate of a cruising ship, and reducing the frictional resistance existing between the water and the hull plate by the presence of the micro-bubbles. Such reduction in the frictional resistance enables to reduce the motive power required for the cruising ship (fuel saving), and ultimately reduce the cost of operating the ship.
This applicant has been proposing a technology for reducing the frictional resistance of a ship based on enveloping a wide area of the ship""s bottom section by jetting air near the bow of the ship from the lateral hull of the ship, which is subjected to lesser static pressure than the bottom hull surface, to generate micro-bubbles and transporting the micro-bubbles along the flowlines to the bottom hull surface. This technology is based on an observation that the flowlines in the vicinity of the ship""s bow swirl from the lateral hull of the ship to the bottom surface of the ship, so that, by jetting a gaseous substance from the lateral hull surface, where the static pressure is less than that at the bottom surface, power required to eject the gas into the water is reduced, thereby controlling the loss of saving the fuel that can be achieved through reducing the frictional resistance.
However, in the conventional technology according to the applicant of this invention, although it was possible to envelope a wide area of the bottom section with micro-bubbles, it was insufficient. That is, there is a problem that although flowlines on the port side and starboard side starting at the bow of the ship do swirl around to the bottom surface from the respective port or starboard side to disperse the micro-bubbles, they are only effective in covering the port and starboard regions of the bottom surface, and are not effective in extending the flow of micro-bubbles to the center section of the bottom surface.
Also, according to the conventional technology, various methods for generating bubbles in the water rely on equipment such as pumps and blowers to pressure the gas, which is then ejected into the water through a plurality of holes provided on the hull or porous plates. However, the method based on ejecting the pressurized gas into the water presents a problem that energy is expended in operating the pressurizing equipment so that it results in a loss of part of the energy saving achieved by reducing the frictional resistance. Especially, if the gas is ejected into the water from relatively deep locations below the surface such as at the bottom surface of large capacity vessels, it is necessary to apply higher pressure in relation to the water pressure (static pressure), thus resulting in expending a large amount of energy. Also, the high cost of pressurizing equipment and facility installation cost must also be considered.
Further, according to the conventional technology, the structure for introducing the gas from the gaseous space into the water is generally achieved by providing a network of piping and ducting throughout the interior of the ship from the equipment to the discharge end. However, the friction reducing ship described above is based on piping networks in the ship interior, the facility must be installed while avoiding interference with other equipment, so that the construction cost tends to be high because of problems such as labor-intensive construction tasks and a large number of parts required for the structure.
This invention is provided in view of the background information presented above, and the objectives of the invention are as follows.
(a) To reduce the motive power required for generating the micro-bubbles.
(b) To envelope a wider bottom surface of a ship with micro-bubbles.
(c) To further improve fuel saving brought about by reducing the frictional resistance.
(d) Use lesser amount of energy in lowering the frictional resistance to lead to reduced energy consumption for cruising.
(e) To mix the bubbles in the water efficiently to achieve effective reduction in frictional resistance.
(f) To reduce the cost of constructing the ship.
To achieve such objects, this invention adopts as the most comprehensive means a friction reducing ship that reduces frictional resistance by ejecting gas bubbles on a submerged surface of a ship body, and adopts as a method for reducing frictional resistance, a means provided by a ship body cruising through a body of water for creating in the water a negative pressure region having a pressure lower than a pressure in a gaseous space, and directing a gas from the gaseous space to the negative pressure region in the water to generate the gas bubbles on the submerged surface.
According to such a structure, no special motive force is required and using a very simple means, it is possible to supply a gas from a gaseous space to the negative pressure region to generate bubble on the submerged surface. Therefore, according to this invention, each of the above objects is achieved.
Also, to achieve the objects in this invention, in a friction reducing ship that reduces frictional resistance by ejecting gas bubbles on a submerged surface of a ship body, a structure is adopted to comprise a bubble generation apparatus for generating micro-bubbles by creating a negative pressure state in a portion of water admitted from a water intake opening provided below a waterline in a bow section of the ship body so as to eject atmospheric air into water and discharging the micro-bubbles together with the water to a water discharge opening provided in a bottom section of the ship.
Also, in a friction reducing ship that reduces frictional resistance by ejecting gas bubbles on a submerged surface of a ship body, a structure is adopted to comprise: a water transport passage provided on an external hull plate so as to extend from a water intake opening provided below a waterline in a bow section to a water discharge opening provided in a bottom section, and having an air discharge opening disposed partway along the passage; an air transport passage extending from above the water to the air discharge opening; and a gas ejection member protruding toward an inner side of the water transport passage and provided in such a way to cover the air discharge opening having a gas ejection opening; wherein the air discharge opening is situated in a location such that a hydrostatic pressure at the air discharge opening is negative with respect to an atmospheric pressure existing above the water.
Furthermore, a method is adopted for reducing frictional resistance of a ship body by creating a negative pressure state in a portion of water admitted from a water intake opening provided in a bow section of the ship body below a waterline so as to generate micro-bubbles of by ejecting atmospheric air into the water and discharging the micro-bubbles together with the water to a water discharge opening provided in a bottom section of the ship.
According to such a friction reducing ship or a method for reducing frictional resistance, a negative pressure region is created in the water admitted from the bow section of the ship so as to discharge the air above the water to the water discharge opening in the bottom section, so that it is possible to generate micro-bubbles in the water without using additional motive power such as compressor for ejecting air into the water. Also, the water intake opening is provided in the bow section and the water discharge opening is provided in the bottom section, by having the ship cruise through a body of water, the water can easily be admitted from the water intake opening and discharged through the water discharge opening. Therefore, micro-bubbles can be generated while suppressing generation of resistance relating to water intake action. Also, because the water discharge opening is located in the bottom section, it is possible to envelope the bottom section effectively with micro-bubbles to reduce frictional resistance effectively, and therefore, to realize improvement in fuel saving due to reduced frictional resistance. Further, the water passage, air passage and the gas ejection member are provided, and the air ejection opening is located in such a way that the hydrostatic pressure is negative with respect to the atmospheric pressure, and therefore, it is possible to generate micro-bubbles within the water using a very simple structure and without using additional motive power such as a compressor. Therefore, this aspect of the invention is also effective in reducing the frictional resistance.
To achieve the objects listed above, a method is adopted for reducing frictional resistance of a ship by ejecting gas bubbles on a submerged surface of a ship body by creating in the water a negative pressure region, having a pressure lower than a pressure in a gaseous space, resulting from the ship body cruising through a body of water, and directing a gas from the gaseous space to the negative pressure region in the water and forming a flow of water having locally severe vortices.
Also, a structure is adopted such that, in a friction reducing ship that reduces frictional resistance by ejecting gas bubbles on a submerged surface of a ship body, a structure comprises: a negative pressure forming section for creating a negative pressure region in water having a lower pressure relative to a gaseous space; a fluid guiding passage for directing a gas from the gaseous space to the negative pressure region; and a detaching promotion section for forming a water flow having locally severe vortices.
The basic principle of this invention will be explained in the following.
In general, when a pressure gradient is formed in a fluid environment, the fluid body is subjected to a pressure gradient force acting in the direction of a higher pressure side to a lower pressure side to induce the fluid to flow. Therefore, by creating a negative pressure region in the water with respect to a pressure in a gaseous space, it becomes possible to send the gas in the gaseous space to the negative pressure region in the water to a certain depth using the pressure gradient force.
FIG. 4 shows a schematic diagram of a friction reducing ship having a negative pressure forming section for creating a negative pressure region in the water. When the ship 1 is moving at a given cruising speed Vs, a water stream 2 relative to the ship is formed. If the water path is narrowed by the negative pressure forming section 3, for example, the velocity of water stream 2 increases, and the hydrostatic pressure Pwa decreases locally (Bernoulli""s principle). Designating the water flow velocity by Vwa, pressure in the gaseous space (atmospheric pressure) by Pa, water density by xcfx81, gravitational acceleration by g, and water depth by Hwa, the hydrostatic pressure Pwa is given by the following expression:
Pwa=Pa+xcfx81xc2x7gxc2x7Hwaxe2x88x92xcfx81xc2x7(Vwa2xe2x88x92Vs2)/2xe2x80x83xe2x80x83(1) 
As can be seen clearly from equation (1), when the water flow velocity Vwa satisfies the next equation, a negative pressure region 4 having a lower pressure relative to the atmospheric pressure is formed as shown in equation (2).
xcfx81xc2x7gxc2x7Hwaxe2x88x92(Vwa2xe2x88x92Vs2)/2 less than 0xe2x80x83xe2x80x83(2) 
When the negative pressure region 4 is formed in the water, the pressure gradient force described above causes the gas to flow inside a fluid passage 5 and is sent into the water. When the gas is sent into the water by forming the negative pressure region 4 (negative pressure method), because there is no need to pressurize the gas, the amount of energy expended in sending the bubbles into the water is less compared with the conventional pressure method. Also, by sending the gas sent into the water as bubbles 6 to mix with the water, numerous bubbles 6 can be located on the submerged surface of the ship body 1, and numerous bubbles 6 are allowed to intervene between the ship body 1 and the submerged surface to reduce the frictional resistance acting on the ship.
However, the bubbles 6 in the water are acted on by many forces. An example of such forces is shown in Table 1 in FIG. 5. For example, as shown in FIG. 4, it is considered that, when the negative pressure forming section 3 is protruding from the bottom section of the ship, the gas flowing inside the fluid passage 5 reaches an interface 7 (gas/liquid interface) between the gas and the liquid (water), and then bubbles 6 transfer into the water as a result of the pressure gradient force exerted by the negative pressure region 4 as well as the lift force, which will be explained later, and then flows by floating in the flow of the water. The lift force is generated when the water stream 2 around the bubbles 6 has eddy current, and the direction of the force is opposite to a vector obtained by a product of a vorticity vector of liquid xcfx89 and a relative velocity vector us. The magnitude is proportional to a bubble volume Av and a liquid density xcfx81. That is, the lift force Lf is expressed by the following equation (3).
Lf=xe2x88x92xcfx81xc2x7Av(usxc2x7xcfx89)xe2x80x83xe2x80x83(3) 
However, the lift force Lf is Auton""s momentum lift force. When the Reynold""s number is low, Staffman lift force operates so that it is proportional to the vortex raised to the power of xc2xd. Here, the direction of action is the same in both cases.
In the boundary layer at the bottom section, streams having vortices are found near the surface of the outer hull plate in general, so that each vector is directed in the direction shown in FIG. 6. As can be understood from FIG. 6, the lift force Lf at the bottom section acts in a direction away from the outer hull plate, that is, in the direction of detaching the bubbles 6 from the gas/liquid interface 7 into the water.
However, depending on the shape of the negative pressure forming section, a relatively large force (resistance force) can sometimes act on the bubbles to force them to return to the gas/liquid interface. For example, when the water flows along the negative pressure forming section 3 shown in FIG. 4, the resistance force on the bubbles 6 is the additional momentum force and the pressure gradient force which will be explained next. The additional momentum force is the momentum force due to the added mass of a bubble placed in the liquid (water), and if the density difference between the gas and liquid is assumed to be {fraction (1/800)}, compared with the momentum force acting on the mass of gas itself residing inside the bubble, it is 400 times larger. Also, compared with the momentum force of water, bubble momentum force+additional momentum force is xc2xd as large. From this, for a given external force, a bubble will generate three times the acceleration of water, (1+1/(xc2xd)=3), assuming the maximum value by neglecting the resistance force.
That is, as shown in FIG. 7, when the bubbles 6 and the water flow along a curved surface of a solid body 8, when the flow direction of water stream 2 changes downward at a -shaped section PA1, the bubbles 6 descends at three times the acceleration of water. Also, when the flow direction of water stream 2 changes upward at a -shaped section PA2, the bubbles 6 ascends at six time the acceleration of water. Therefore, when the water flows along the negative pressure forming section 3 in FIG. 4, because of the curvature of the apex section of the negative pressure forming section 3 (-shaped section), as the stream 2 changes the flow direction to upward direction at the negative pressure forming section 3, additional momentum force operates to push the bubble back to the gas/liquid interface 7.
Also, in the case shown in FIG. 4, because the pressure at the negative pressure region 4 is lower compared with other regions in the water, the bubbles 6 residing in the negative pressure region 4 is acted by the pressure gradient force to push it back to the gas/liquid interface 7. Then, when a large force (resistance force) to push to the gas/liquid interface acts on the bubble, the bubble becomes more difficult to be detached from the gas/liquid interface so that the lesser amount of bubbles are mixed in the water, and there is a danger that the frictional resistance may not be reduced effectively.
Therefore, the flow of water is formed so as to facilitate the movement of the bubbles from the gas/liquid interface into water so as to decrease the resistance force acting against detachment of the bubbles, the bubbles are facilitated to leave the gas/liquid interface to increase the amount of bubbles mixed in the water. That is, by forming the flow so as to generate high degree of local vortices, the lift force acts on the bubbles in the direction of detachment to promote detachment of bubbles from the gas/liquid interface.
According to the method of reducing frictional resistance based on the principle outlined above, by forming negative pressure regions in the water, reduction of frictional resistance can be carried out utilizing the pressure gradient force, with a lesser amount of energy to send the air into water, compared with the case of compressing the air. It follows that effective friction resistance reduction is achieved to reduce the energy consumption during cruising.
Also, according to the friction reducing ship of this invention, by having the negative pressure forming section to form a negative pressure region in the water, reduction of frictional resistance can be carried out utilizing the pressure gradient force, with a lesser amount of energy to send the air into water, compared with the case of compressing the air. Also, using the flow formed by the detaching promotion section, detaching of bubbles from the gas/liquid interface is promoted by utilizing the lift force to increase the amount of bubbles mixed in the water. It follows that effective reduction in frictional resistance is achieved to reduce the energy consumption during cruising. Further, equipment for pressurizing the air becomes unnecessary so that the cost of building the ship can be lowered.
Further, to achieve the objects (d)xcx9c(f), in a friction reducing ship that reduces frictional resistance by ejecting gas bubbles on a submerged surface of a ship body, a structure is adopted to comprise: a negative pressure forming section protruding from the submerged surface for creating a negative pressure region in water relative to a gaseous space; a discharge opening for ejecting gas bubbles towards the negative pressure region in water; a fluid passage having one end open to the gaseous space and having other end open in water by way of the discharge opening so as to direct a gas from the gaseous space into water; wherein the discharge opening is disposed on an inclined surface inclined at an angle to the submerged surface of the ship body.
According to such a friction reducing ship, because a negative pressure region that has a pressure lower than the gaseous space is formed by the negative pressure forming section, the gas is directed into the water through the fluid passage by utilizing the pressure gradient force, and the bubbles are discharged into water from the water discharge opening. Because the water discharge opening is disposed in the inclined surface inclined at an angle to the submerged surface of the ship body, the area of the discharge opening can be enlarged readily within a given region of the submerged surface. Further, because the inclined surface is disposed in a depression provided on a submerged surface of a ship body to extend from an inner location to an outer location of the depression, by having at least a portion of the discharge opening within the depression, the height of protrusion of the inclined surface from the submerged surface of the ship body can be controlled even when a large area is provided for the discharge opening. Therefore, a large amount of bubbles can be ejected from the discharge opening of a large opening area while suppressing any increase in the resistance force against the flow of water.
As a result, it is possible to reduce the frictional resistance of a ship using a lesser amount of energy by sending the gas into the water by utilizing the pressure gradient force compared with the case of pressurizing the air. Also, because the discharge opening for ejecting bubbles is disposed on an inclined surface disposed at an angle to the submerged surface of the ship body and the inclined surface is provided to extend from the inside of the depression disposed on the submerged surface to external section, a large amount of bubbles can be ejected from the discharge opening of a large opening area while suppressing any increase in the resistance force against the flow of water. Therefore, using a large amount of bubbles to achieve an effective reduction in frictional resistance to reduce energy consumption for cruising operation. Also, apparatus for pressurizing the gas is not necessary so that the cost of building the ship can be readily reduced.
Further, to achieve the objects (d) and (e) in this invention, in a friction reducing ship that reduces frictional resistance by ejecting gas bubbles on a submerged surface of a ship body, a structure is adopted to comprise: a negative pressure forming section protruding from the submerged surface for creating a negative pressure region in water relative to a gaseous space; a discharge opening disposed in a rear of the negative pressure forming section for ejecting gas bubbles towards the negative pressure region in water; a fluid passage having one end open to the gaseous space and having other end open in water by way of the discharge opening so as to direct a gas from the gaseous space into water; and a gas supply apparatus for supplying a gas towards the negative pressure region.
Also a method for reducing frictional resistance by ejecting gas bubbles on a submerged surface of a ship body by creating in the water a negative pressure region, having a pressure lower than a pressure in a gaseous space, resulting from the ship body cruising through a body of water, and directing a gas from the gaseous space to the negative pressure region in the water so as to eject bubbles into a body of water, and supplying a gas to the negative pressure region by using a specific apparatus.
According to such a friction reducing ship or a method for reducing frictional resistance of a ship, because a negative pressure region that has a pressure lower than the gaseous space is formed by the negative pressure forming section, the gas is directed into the water through the fluid passage by utilizing the pressure gradient force. Also, by using the gas supply apparatus to supply a gas into the fluid passage, the amount of gas flowing in the fluid passage is increased, and a large amount of bubbles are ejected from the discharge opening. A result is that, by disposing a portion of the negative pressure forming section to protrude from the submerged surface of the ship body, it is possible to reduce the frictional resistance of a ship using a lesser amount of energy by sending the gas into the water by utilizing the pressure gradient force compared with the case of pressurizing the air. Also, by supply the gas from the gas supply apparatus, the volume of gas flowing in the fluid passage can be increased with a small amount of energy so as to discharge a large amount of bubbles from the discharge opening. Therefore, effective reduction in frictional resistance is achieved using a large amount of bubbles to reduce the energy consumption during cursing operation.
Also, to achieve the objects (d)xcx9c(f) in this invention, a method is adopted for reducing frictional resistance by ejecting gas bubbles on a submerged surface of a ship body by creating in the water a negative pressure region, having a pressure lower than a pressure in a gaseous space, resulting from the ship body cruising through a body of water, and directing a gas from the gaseous space to the negative pressure region in the water, and generating a circulating flow of water by using a wing to expand the negative pressure region.
In a friction reducing ship that reduces frictional resistance by ejecting gas bubbles on a submerged surface of a ship body, a structure is adopted to comprise: a negative pressure forming section protruding from the submerged surface for creating a negative pressure region in water relative to a gaseous space; a discharge opening disposed in a rear of the negative pressure forming section for ejecting gas bubbles towards the negative pressure region in water; a fluid passage having one end open to the gaseous space and having other end open in water by way of the discharge opening so as to direct a gas from the gaseous space into water; wherein the negative pressure forming section is provided with a wing shaped component whose cross sectional shape is formed in a wing shape.
According to such a friction reducing ship or a method for reducing frictional resistance of a ship, because a negative pressure region that has a pressure lower than the gaseous space is formed by the negative pressure forming section, the gas is directed into the water through the fluid passage by utilizing the pressure gradient force. In this case, by developing the negative pressure region by creating a circulating flow in the water due to the wing, the pressure gradient force is increased to direct a large amount of bubbles into the water. Also, by applying an uplifting force on the ship body due to the circulating flow, it is possible to reduce the area of the submerged surface of the ship body. A result is that, by disposing a portion of the negative pressure forming section to protrude from the submerged surface of the ship body, it is possible to reduce the frictional resistance of a ship using a lesser amount of energy by sending the gas into the water by utilizing the pressure gradient force compared with the case of pressurizing the air. Also, by developing the negative pressure region in the water due to the circulating flow caused by the wing, it is possible to increase the amount of bubbles mixed in the water. Further, by applying an uplifting force on the ship body due to the circulating flow, it is possible to reduce the area of the submerged surface of the ship body. effective reduction in frictional resistance is achieved using a large amount of bubbles to reduce the energy consumption during cursing operation.
Further, to achieve the object (f) in this invention, in a friction reducing ship that reduces frictional force by ejecting gas bubbles on a submerged surface of a ship body, a structure is adopted to comprise: a discharge opening disposed on the submerged surface for ejecting gas bubbles into water; a fluid passage having one end open to the gaseous space and having other end open in water by way of the discharge opening so as to direct a gas from the gaseous space into water; wherein at least a portion of the fluid passage is comprised by component members to form outer shell of the ship body.
According to such a friction reducing ship, because the fluid passage is constructed of the member comprising the outer shell plate of the ship body, there is no need to install new piping for directing a gas into the water, thus enabling to reduce the cost of building the ship by reducing the number of members and labor cost. Also, the ducts that form the fluid passage are reinforcing members of the ship body, and therefore, a high degree of design freedom is possible such as providing many ducts on the ship body and disposing the ducts wherever needed. Furthermore, by forming the fluid passage in a divided manner, a plurality of fluid passages may be made available for selection.